Many raptors are monomorphic in plumage but sexually dimorphic in size, with females being relatively larger than males (Andersson and Norberg 1981, Krüger 2005). Female Bald Eagles (Haliaeetus leucocephalus) can be about 25% larger than males (Buehler 2000). Accurate identification of sex is an important factor in the conservation, management, and understanding of a species' ecology. For example, ignoring sex may bias estimates when examining variables such as home-range size, migration, food preferences, and behavior (Andersson and Norberg 1981). The ability to distinguish sex on initial handling would greatly facilitate study designs which require sampling of equal ratios of males and females.

Several techniques are available to determine sex of birds. Molecular techniques use DNA to accurately determine the sex of a species (Au et al. 1975). However, these results often are obtained post-release and can be costly when...

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